1. Field of the Invention
The present invention relates to an R-T-B (rare-earth-iron-boron) based sintered magnet.
2. Description of the Related Art
R-T-B based sintered magnets have so good magnetic properties as to find a wide variety of applications including various types of motors and actuators and are now one of indispensable materials for the electronics industry. Also, their applications have been appreciably broadened to keep up with the recent trend toward energy saving.
Lately, however, those motors and actuators are more and more often required to exhibit much higher performance than conventional ones in their rapidly expanding applications including motors for driving, or generating electricity for, hybrid cars or motors for hoisting elevators. And their requirements are becoming increasingly severe nowadays.
One of the old drawbacks of R-T-B based magnets is their relative low Curie temperature of approximately 300° C., at which their ferromagnetism is lost. And their coercivity varies so significantly according to the temperature that irreversible flux loss will occur easily. To overcome such a problem, various measures have been taken. For example, some people tried to increase the coercivity of the R-T-B based magnets by adjusting the combination of rare-earth elements to add. Other people attempted to increase the Curie temperature by adding Co as disclosed in Patent Document No. 1. However, none of these measures will be effective enough to reduce the significant variation in coercivity with the temperature.
Several methods for increasing the coercivity have been proposed so far.
One of those methods is disclosed in Patent Document No. 2, in which heavy rare-earth elements such as Dy and Tb are included in particular percentages in the rare-earth elements. In practice, only Dy and Tb turned out to be effective enough. This method is adopted in order to increase the coercivity of the magnet as a whole, as well as the anisotropic magnetic field of its main phase that determines its magnetic properties. However, those heavy rare-earth elements such as Dy and Tb are among the rarest and most expensive ones of all rare-earth elements. For that reason, if a lot of such heavy rare-earth elements should be used, then the price of the magnets would rise. In addition, as the applications of such R-T-B based sintered magnet have been rapidly expanding these days, resource-related restrictions on those heavy rare-earth elements have become an issue these days because those rare elements are available only in very limited quantities and in very narrow areas.
Another method is disclosed in Patent Documents Nos. 3 and 4, for example, in which the coercivity is increased by introducing an additive element such as Al, Ga, Sn, Cu or Ag. It is not yet quite clear exactly how these elements can increase the coercivity. Nevertheless, it is at least known that the coercivity can be increased by changing the physical properties of a grain boundary phase (which is a so-called “R-rich phase”) such as its wettability with the main phase in a high temperature range and eventually changing the microstructures with the addition of those elements. It is also known that those elements can relax the heat treatment conditions in order to increase the coercivity. However, Al, for example, could form a solid solution even in the main phase of the magnet. That is why if the amount of such an additive were increased, the Curie temperature and magnetization of the main phase would decrease, which is a problem.
Furthermore, the additive elements such as Ti, V, Cr, Zr, Nb, Mo, Hf and W disclosed in Patent Document No. 5, for example, hinder the growth of crystal grains during the sintering process and reduce the size of the resultant metallurgical structure of the sintered body, thus contributing to increasing the coercivity.
Among these methods, the method that uses heavy rare-earth elements is most effective because the decrease in magnetic flux density is relatively small according to that method. According to any of the other methods mentioned above, however, a significant decrease in the magnetic flux density of the magnet is inevitable. And those methods are applicable to only a narrow field. For that reason, in making magnets actually, these techniques are used in an appropriate combination.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 59-64733        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 60-34005        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 59-89401        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 64-7503        Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 62-23960        
In the prior art, the compositions of magnets have actually been determined by adopting those techniques in an appropriate combination to realize required good magnetic properties (and desired high coercivity, among other things). Nevertheless, there is a growing demand for magnets with even higher coercivity.
An object of the present invention is to provide means for increasing the coercivity effectively with the decrease in magnetization minimized and without always using a heavy rare-earth element such as Dy or Tb.